Is blender actually useable for engineering?
TL:DR Blender is a weak app for actual engineering work, it's not a CAD software. - Instead try free CAD apps like FreeCAD, NaroCAD, SolveSpace, DesignSpark Mechanical, OpenFoam for CFD and I am sure many others.
I can recommend Blender for modeling objects to which you already know their design and parameters and you are sure the shape is mostly final (like modeling your living room or a part that broke and you need to 3D print it). Good for organic modeling through sculpting too. Blender is an app designed for animation studios and entertainment industry. Engineering workflow in Blender is slow and full of workarounds. You'll be fine with simple shapes, but not with engineering anything slightly more complex.
The bad:

There are no work planes
The modeling workflow is generally destructive
The procedural stuff (modifier stack, object nodes,..) has big limitations
You cannot easily constrain geometry elements together (parallel, tangent, etc. relationships)
NURBS or parametric surface tools are practically non-existent, so everything will be made from polygons. This can bring whole lot of problems you don't want to deal with.
No easy export into technical drawings

The good:

There is modifier stack providing functionality like Mirror, Array, Revolve, Boolean, etc. The stack itself is like modeling history tree. But some of these modifiers do fail if they encounter non-manifold mesh or flipped polygon normals. It's not exactly fool-proof.
You can tag measurements with MeasureIt addon
Blender is quite capable with 3D printing your stuff with the help of 3D Print Toolbox addon
With Sverchok addon (or Object nodes that are coming into Blender) you can create geometry procedurally - it is similar to Rhino's grasshopper
Visualization of your models is top notch, you can go photoreal